Trump adds more heat to NFL 'kneeling' protest

US President Donald Trump once again lashed out at the National Football League, adding heat to brewing controversy over the right of players to hold symbolic protests.

His early morning Twitter barbs came hours after the Dallas Cowboys, along with coaches and the team owner, collectively took a knee before their game Monday night in what they called a statement of equality and unity.

The president has been locked in a days-long feud with NFL players, owners and league officials over the symbolic gesture intended to draw attention to racial injustice in the United States.

"The booing at the NFL football game last night, when the entire Dallas team dropped to its knees, was loudest I have ever heard. Great anger," Mr Trump tweeted.

"Ratings for NFL football are way down except before game starts, when people tune in to see whether or not our country will be disrespected!"

"The NFL has all sorts of rules and regulations. The only way out for them is to set a rule that you can't kneel during our National Anthem!" he added.

Some opposing fans from Arizona booed the Cowboys when they took a knee on Monday night in Phoenix.

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The US president did note that Dallas players stood when "The Star Spangled Banner" played.

"While Dallas dropped to its knees as a team, they all stood up for our National Anthem. Big progress being made -- we all love our country!"

Over the weekend more than 150 professional players from across the country's most popular sport took a defiant stance, kneeling, linking arms or raising clenched fists during the anthem.

Mr Trump had ignited tumult by attacking players like Colin Kaepernick -- who first took a knee through renditions of the US national anthem during last year's American football season -- as a "son of a bitch" who should be fired.

A number of owners and the NFL commissioner have issued statements objecting to Mr Trump's comments.

And Nike, an NFL sponsor, expressed "support" for players in a brief statement obtained by AFP.

"Nike supports athletes and their right to freedom of expression on issues that are of great importance to our society," the brand said.

In a separate feud, Mr Trump also withdrew a White House invitation for the NBA champion Golden State Warriors and star player Stephen Curry.

Under Armour, which endorses Curry, weighed in on the controversy over the weekend, saying it "stands for the flag and by our Athletes for free speech, expression and a unified America."

The Dallas Cowboys take a knee prior to the national anthem prior to an NFL football game against the Arizona Cardinals. (AAP) ()Oakland Athletics' Mark Canha (20) places his hand on the shoulder of Bruce Maxwell as Maxwell kneels during the national anthem for the third consecutive day, prior to the team's baseball game against the Seattle Mariners. (AAP) ()